The literature in the pyrolysis art frequently uses terminology which is incorrect or misleading. For purposes of explaining this invention, some general understandings with respect to the terminology used herein and as used in the claims hereof will have the meanings set forth herein. "Pyrolysis", in a technical sense, means the chemical decomposition or change in a material brought about by heating the material in the absence of oxygen. In the commercial application of pyrolysis, this cannot occur because of oxygen or air leakage inherent in commercial furnaces used to pyrolyze waste material. To prevent leakage of fumes from the furnace into the work place, pyrolyzing furnaces are typically operated at a slight negative pressure which results in air being drawn into the furnace. Typically, the furnace pressure is controlled so that no more than about 2% of the furnace atmosphere during the pyrolyzing stage is oxygen. "Pyrolysis" as thus used herein does contemplate that a slight percentage of oxygen will be present in the furnace atmosphere during the process. Accordingly, the specifications hereof will discuss the pyrolysis process along the classical heat transfer lines of whether or not the thermal reactions occurring within the furnace are endothermic or exothermic. The atmopshere within the furnace or the fumes which are drawn off from the furnace will then be discussed with reference to the percentage of the volatiles present in the fumes, it being understood that when the furnace is operating in the pyrolyzing mode, the oxygen content of the fumes emitted from the furnace is a very slight amount and is maintained at a level which will not permit the fumes to have an oxygen content usch that the mixture is combustible. This occurs only during the pyrolyzing mode.
Other terms which tend to be confused in the art include "stoichiometric" and "starved air" or "starved combustion". "Stoichiometric" is technically defined as an adjective characterized by being that portion of substances exactly right for a specific chemical reaction to occur with no excess of any reactant or product. The term "stoichiometric" is typically used in the burner art to mean that metered amounts of fuel and combusion air are supplied to the burner so that the fuel is completely combusted by the precise amount of air provided. "Starved air" or "starved combustion" means that the air or oxygen is supplied at a rate which is less than stoichiometric when compared to the amount of oxygen required for stoichiometric combustion of the material. Arbitrarily, starved air means oxygen supplied at a rate equal to anywhere from 40-99% of the oxygen required to achieve stoichiometric combustion. Having thus defined such terms, the definitions are admittedly of slight value because under the starved air mode an endothermic reaction can become exothermic as a function of time because over a fixed time period a given quantity of oxygen will be supplied to the reactants. The definitions are nevertheless helpful to distinguish incinerator apparatus operated in a starved air mode and erroneously referred to as a pyrolyzer. Reference may be had to U.S. Pat. No. 4,649,834 to Heran describing a water activated temperature control system for a pyrolyzer which, in fact, appears to be a furnace operated under starved air conditions. Reference may also be had to U.S. Pat. Nos. 4,474,121 and 4,517,906 to Lewis which discuss starved air combustion in terms of stoichiometric relationships and identifies the pyrolysis misnomer applied to such processes. Such distinctions become significant when considering the control aspects of the present invention.
Insofar as pyrolyzing processes are concerned, the present inventors have developed and perfected for batch type pyrolyzing furnaces a two-step process. The process comprises an endothermic first step where pyrolysis occurs followed by an optional "burnout" step which incinerates or burns the residue or char remaining from the waste after pyrolysis. The endothermic step is generally conducted at temperatures between 250.degree.-1400.degree. F. and the exothermic step is generally conducted at temperatures between 1400.degree.-2500.degree. F. This is the general pyrolysis batch process as conventionally practiced by the inventors and includes an afterburner for combusting the volatiles distilled from the waste during the pyrolyzing step. As alluded to above, the reason for dividing the process into two steps is to permit the endothermic step to be controlled. In the starved air systems discussed above, the reactions which are both exothermic and endothermic cannot be controlled. This is the reason for many of the control schemes present in the prior art which are then necessary to prevent the waste material from generating high rates of heat and producing a "runaway" situation which can easily result in an explosion.
In the published art, it is generally accepted that pyrolysis is defined as a two-step process in the sense that waste is pyrolyzed in a first step and the fumes or volatiles emitted from the waste are combusted in an afterburner in a second step. The burnout step is generally not practiced or, if practiced, there is no significant distinction or accommodations made in the equipment to handle the exothermic reaction.
Insofar as controlling the endothermic reactions, the inventors have developed for use in their pyrolyzing batch furnaces, a concept defined herein as "signature heat profile". Insofar as it is pertinent to the discussion of the prior art as practiced by the inventors, it is known to take a sample of a waste specimen and pyrolyze the specimen at various temperatures while recording the weight loss of the specimen in a gravimetric furnace until volatilization is achieved in optimal processing times. The time-temperature graph for the specimen, i.e. the heat profile, thus obtained in the gravimetric furnace then becomes the "signature" which is programmed into the commercial pyrolyzing furnace for treating that particular waste. In this manner, batch pyrolyzing of complex, heterogeneous waste material (including many hazardous and toxic substances) containing competing reactions has been successfully accomplished. Because of variations in the waste in commercial applications, the inventors have developed a control arrangement for single pyrolyzing batch furnace applications where a specific type of prior art incinerator (described in patents incorporated by reference herein) is used to incinerate the fumes and the temperature of the incinerator gas is utilized as the only control to check the progress of the pre-programmed signature profile. Specifically, when the temperature of the incinerator gases begin to rise above a predetermined level, the pre-programmed heat profile is interrupted and the burner firing rate retarded at the pyrobatch furnace until the process is under control at which time the profile is reactivated. This arrangement produced a very simple control concept which has been successfully demonstrated in commercial applications involving an afterburner connected to a single batch furnace. The signature heat profile concept gradually evolved over a period of several years by the inventors and ramifications of the concept are still being made, one of which forms a feature of this invention and will be described in detail hereafter.
While there are virtually a countless number of afterburners or incinerators which have been used in the prior art to incinerate the fumes given off in the pyrolysis process, the inventors have heretofore used for their batch furnace applications a particular incinerator of the type described in U.S. Pat. No. 3,838,974, incorporated by reference herein. In that arrangement, a jet pump annulus of cold combustion air pulls the fumes from the pyrolyzer into a combustion chamber whereat the jet expands into contact with the combustion chamber walls to produce turbulence. The turbulence causes mixing of the fumes and the combustion air to produce a mixture capable of sustaining combustion which is stabilized, ignited and combusted in the incinerator. Burners are added to the combustion chamber to ignite the mixture during start-up and afterwards to supply metered amounts of air or combustibles to maintain the mixture being incinerated at the desired temperature. The temperature variation of the incinerator gases as sensed by a thermocouple in the rich fume incinerator is thus a function of the volatile content of the fumes emitted from the pyrobatch furnace in contrast to other afterburner control arrangements. Thus, one temperature sensor which controls the operation of the incinerator also insures that the pyrolysis in the batch furnace is proceeding in the proper manner.
While the batch furnace, as thus described, has successfully operated to optimally process complex and rather exotic toxic and/or hazardous waste materials in short time periods, each pyrolysis batch furnace required a rich fume incinerator, a control arrangement, and associated pollution control equipment. Commercial waste treating facilities dispose of many types of hazardous waste. Treating a variety of waste means different signature profiles which result in the batch furnaces processing loads smaller than that desired for optimal equipment utilization. Preferably, for industrial and commercial waste treaters small size batch furnaces are preferred. However, the equipment cost begins to significantly rise not only because each furnace requires its own incinerator, but also, because each incinerator requires its own pollution control equipment such as scrubbers and the like. Heretofore, it was not possible to plumb several batch furnaces into one incinerator with its related equipment simply because if a burnout step was occurring in any one furnace, streams containing air would mix with volatiles in other streams producing an explosive mixture. Secondly, considering systems that operate only in the pyrolysis stage without any burnout, the afterburner and pollution control equipment would have to be sized for the total cumulative furnace load the afterburner would be exposed to, and while the capital cost of one large afterburner compared to many small ones would be reduced, the equipment cost is still significant.